1. Field of the Invention
The present invention relates to a sensor (an MEMS sensor) manufactured by an MEMS (Micro Electro Mechanical Systems) technique.
2. Description of Related Art
An MEMS sensor, having been recently loaded on a portable telephone or the like, is increasingly watched with interest. For example, a piezoresistive acceleration sensor for detecting the acceleration of a substance is known as a typical MEMS sensor.
A conventional piezoresistive acceleration sensor includes a frame in the form of a quadrilateral frame, a beam supported by the frame, and four weights supported by the beam. The beam is cruciform in plan view, and extended between central portions of the respective sides of the frame. Piezoresistive elements and wires are provided on the beam. The weights, provided on four regions partitioned by the frame and the beam, are in the form of quadrilateral poles having a thickness generally identical to that of the frame. One corner portion of each weight arranged closer to the central side of the frame is connected to the cross portion (the central portion) of the beam while the remaining portions are separated from the frame and the beam, so that the weight is supported by the beam in a vibratile manner.
When acceleration acts on the piezoresistive acceleration sensor and the weights are vibrated, the beam is distorted. Thus, stress is applied to the piezoresistive elements provided on the beam, to change the resistance values thereof. When the quantity of the change in the resistance value of each piezoresistive element is extracted as a signal, therefore, the acceleration acting on the piezoresistive acceleration sensor can be detected on the basis of the signal.
The piezoresistive acceleration sensor having the aforementioned structure is manufactured by employing an SOI substrate having a multilayer structure of a silicon base layer having a thickness of 400 μm, a silicon oxide layer having a thickness of 1 μm and a silicon surface layer having a thickness of 7 μm, for example. In the steps of manufacturing the piezoresistive acceleration sensor, the silicon surface layer is first selectively etched with the silicon oxide layer serving as an etching stopper, so that surface-side grooves surrounding portions for forming the weights respectively are formed in the silicon surface layer. Then, the silicon base layer is etched with the silicon oxide layer serving as an etching stopper while leaving portions for forming the frame and the weights, so that rear-side grooves are formed in the silicon surface layer. Then, portions of the silicon oxide layer exposed through the rear-side grooves are etched, whereby the frame and the weights are separated from one another, and the piezoresistive acceleration sensor is obtained.
In order to separate the frame and the weights from one another, however, the steps of forming the surface-side grooves and the rear-side grooves must be separately carried out, to disadvantageously require a long time for manufacturing the piezoresistive acceleration sensor. Further, the silicon base layer having a relatively large thickness must be etched over the thickness direction thereof in order to form the rear-side grooves, and the etching (formation of the rear-side grooves) also requires a long time.